xsession: source /etc/profile at the beginning of the script

The xsession script runs services that depend on a sane environment. Gpg-agent, for
example, runs the program "pinentry-gtk-2" to obtain the password to unlock GnuPG
and SSH keys. That program will display only gibberish unless $FONTCONFIG_FILE is
configured properly. Instead of configuring these variables explicitly one by one,
we just source /etc/profile, which contains the appropriate @shellInit@ code.